This invention relates to a magnetic metallic powder of the type obtained by reduction of an iron oxide powder, which may optionally contain cobalt, and a magnetic recording medium utilizing this magnetic metallic powder.
In the production of magnetic recording media such as magnetic tapes, selective use is made of magnetic powder materials having relatively large values for both residual magnetization and coercive force. Recently, higher recording density and lower noise have been increasingly required of magnetic tapes, wherefore there is an unbounded demand for magnetic powder materials of extremely fine particle size and superior magnetic properties.
In such circumstances, it has been endeavored to obtain a ferromagnetic iron (essentially) powder which is satisfactorily large in magnetization per unit mass and especially high in coercive force by reduction (predominantly by heating in a reducing atmosphere typified by a hydrogen gas atmosphere) of an iron oxide powder such as maghemite (.gamma.-Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4), goethite (FeOOH) cobalt-containing maghemite, cobalt-containing magnetite or cobalt-containing goethite.
To realize required levels of high recording density and low noise when used in magnetic tapes, a magnetic iron powder (or iron-cobalt powder) must be very fine in particle size and anisotropic in particle shape besides a large magnetization value per unit mass and a high coercive force. In the case of producing a magnetic iron powder by reduction of an iron oxide power, it is a requisite that the iron oxide powder, too, is very finely divided and has shape anisotropy since not only the particle shape and size but also magnetization and coercive force of the obtained iron powder depend on the shape and size of the iron oxide particles.
Regarding the production of a ferromagnetic iron powder by heat reduction of a finely divided iron oxide powder, a problem awaiting solution is that the iron oxide particles tend to undergo sintering, i.e. agglomeration of the particles, during the reduction process particularly when the iron oxide particles are smaller than about 0.5 .mu.m in their major axis length. This phenomenon makes it difficult to obtain a magnetic iron powder as very finely divided particles of an expected shape and accordingly with desired magnetic properties.
In the present application a magnetic powder produced by reduction of an iron oxide powder or iron-cobalt oxide powder will be called "metallic powder" because, as is commonly recognized, the product usually contains a certain amount of oxygen depending on the extent of the reduction.